The present invention relates to a control apparatus for controlling a measuring unit of an internal combustion engine wherein an intake airflow rate in an intake pipe of the engine is measured with a heat-wire type airflow rate measuring unit and the measurement signal obtained is used as a signal for detecting the operating state of the engine and, more particularly, to a control apparatus wherein dust attached to a heat-wire type heat-generating element used as a measuring element of the measuring unit is effectively burned off so that airflow rate measurement precision is improved.
When an internal combustion engine is electronically controlled, the operating state of the engine is monitored. A signal corresponding to the operating state is detected. The fuel injection quantity, ignition timing or the like is calculated in accordance with the detection signal and operation control of the engine is thus performed.
The monitor means for monitoring the operating state of an internal combustion engine includes a rotational speed sensor for the engine, an exhaust gas temperature sensor, and a throttle opening sensor. Among various sensors, an intake airflow rate measuring unit is directly related to the operating state of the engine. An airflow rate sensor using a heat wire is known as one type of intake airflow rate measuring unit. In this sensor, a heat-generating element with a controlled heat-generating function is arranged in an intake pipe. Changes in temperature of the heat-generating element which correspond to the airflow rate are measured.
The heat-generating element used in such a heat-wire type airflow rate measuring unit comprises a resistive element which changes its resistance in accordance with a change in temperature. A heating current is supplied to the resistive element so that the element heats to a predetermined temperature. In this case, the amount of heat generated by the element is changed in accordance with the rate of air flowing in the intake pipe. Therefore, the airflow intake rate can be measured by monitoring the temperature of the heat-generating element in accordance with the heating current and the resistance of the heat-generating element.
However, in such a heat-wire type airflow rate measuring unit, when the heat-generating element is exposed in an airflow for a long period of time for measurement purpose, dust in the air deposits on the element and the thermal conductivity of the element is unavoidably changed. In such a state, a change in resistance of the element does not correctly reflect an airflow rate, and the airflow rate measurement signal includes an error component.
In view of this problem, it has been proposed to burn off the dust attaching to the element by increasing the heating temperature of the heat-generating element. As described in Japanese Patent Disclosure No. 54-76182, for example, the equivalent conditions of a bridge circuit connected to the heat-generating element are changed so as to increase the preset heating temperature of the element. However, since the constants of constituent elements of the bridge circuit for monitoring the temperature of the heat-generating element must be changed for this purpose, a circuit and a signal line are required. This results in an adverse effect on the measuring circuit due to unstable factors such as noise and in a low measurement precision of the airflow rate.